Fluorescent pigments are will known in commerce and have many uses. Pigments prepared from a frangible polymer into which a fluorescent dye has been dissolved are particularly well known. The dyes are usually those which fluoresce in daylight to produce very brilliant colors. They are widely used in inks and coatings, especially for outdoor signs. U.S. Pat. Nos. 5,470,502; 3,922,232; 3,812,051 and 3,812,053 are but a few patents that disclose these types of fluorescent dyes.
In recent years, there has been a great deal of interest in methods for marking various products and substrates in a way that the markings are poorly visible or invisible in the visible wavelength, i.e., 400 nm to about 700 nm but which may be detected by means of their fluorescence when they are excited or irradiated with an energy in the near infrared or infrared wavelength region. For example, U.S. Pat. No. 5,093,147 describes an ink containing dyes which are poorly absorptive of radiation in the visible range of about 400 nm to 700 nm but are highly absorptive of radiation in the near infrared range of at least about 750 nm. Those materials which absorb radiation in the near infrared range of 650-1100 nm and fluoresce at a somewhat longer wavelength than that at which they absorb are referred to as near infrared fluorophores.
U.S. Pat. No. 5,525,516 discloses a method for imparting invisible markings for identification purposes to petroleum hydrocarbons using one or more infrared fluorophores selected from the classes of phthalocyanines, squaraines and naphthalocyanines.
Near infrared fluorescing compounds may be mono or multi-functional and may be copolymerized into a variety of condensation polymers. Suitably functionalized near infrared fluorescing compounds may be incorporated into certain polyesters to render the near infrared fluorescing compounds water dispersible. For example, U.S. Pat. No. 5,614,008 discloses an ink composition having a water dissipatable polyester having a near infrared compound copolymerized in the polyester. The ink is useful for invisibly marking a variety of substrates.
U.S. Pat. No. 5,461,136 discloses a method for tagging thermoplastic containers using a near infrared fluorescing compound that is admixed or copolymerized with the thermoplastic material. The incorporation of such near infrared fluorescing compounds into the thermoplastic assists in the separating and sorting of thermoplastic containers such as bottles.
Accordingly, there is a need for a pigmentary form of a near infrared fluorescing compound that may be used in various printing applications and ink formulations.